The present invention relates generally to circuit board construction, and more particularly relates to apparatus and methods for operatively connecting an electronic component to a circuit board using a ball grid array ("BGA") on the electronic component.
For many years it has been customary to employ printed circuit boards ("PCBs") or printed circuit assemblies ("PCAs") as mediums for mechanically holding electronic components together and providing operative electrical interconnections between the components. The earliest PCBs were constructed of an insulating planar substrate (such as a glass fiber/resin combination) upon which a layer of conductive metal was deposited. The metal coating layer typically covered the entire surface of the substrate and was subsequently etched by a chemical process to form from the initial metal coating layer a predetermined pattern of conductive "traces" on the side surface of the substrate. Often, these electrically conductive traces were formed on both sides of the substrate to allow conductors to cross without coming into contact with one another. A plurality of mounting holes or "vias" were drilled through the metal layer(s) and the substrate, and were appropriately positioned to receive leads from the electronic components. This method of mounting electrical components on a circuit board is commonly referred to as "through-hole" mounting.
To complete assembly of a circuit board, the electronic components were placed on the PCB, either by hand or robotic machine, with the leads of the components passing through corresponding vias. Finally, solder connections were made to ensure reliable electrical contact between the components and the traces.
Initially, soldering was performed manually. Subsequently, more efficient machine-soldering techniques employing infrared ovens or solder baths were developed to speed manufacture of circuit boards and to ensure higher solder joint reliability. Under such machine-soldering techniques, the PCB and its components were heated while solder, under the influence of flux, was caused to contact the board and flow by capillary action into the vias, yielding a low resistance solder joint when cooled.
As circuit board technology developed, designers began to create circuit boards comprising many alternating substrate and conductive layer pairs, resulting in sandwiched circuit boards that could accommodate a higher component density. Such boards could accommodate ten or more conductive layers. Later, surface-mount technology allowed the leads to be soldered to solder pads on the surface of the circuit board, rather than requiring the leads to pass through vias to be soldered therein.
In addition to this circuit board construction evolution, the electronic components themselves underwent changes to accommodate higher density. First, discrete components were combined into integrated circuits ("ICs"). ICs were originally placed in dual in-line packages ("DIPs") each consisting of an elongated plastic body encapsulating the IC and a plurality of electrical leads coupled to the IC and arranged in a series extending from the two long edges of the body. The leads could either be through-hole soldered or surface-mounted. Unfortunately, the number of leads that a DIP could accommodate was a function of twice the length of the DIP body edges. Some improvement was made by providing packages having leads extending from all four edges of the body, but, even so, the number of leads was a function of the perimetral length of the body edges.
Next, in an effort to increase lead density further (to address, in particular, the increasing power and density of microprocessors and the stringent space requirements of notebook, subnotebook and personal digital assistant ("PDA") computers), designers developed quad flat packs ("QFPs") comprising a generally square body having leads extending downward from the lower surface of the body. The leads were typically arranged in multiple rows and columns, allowing the QFPs to accommodate more pins than DIPs. However, limitations in socket size and collective lead insertion force began to be problematical.
Currently, designers are focussing on ball grid array ("BGA") packaging wherein leads are dispensed with and replaced with a finely-pitched matrix of conductive contact surfaces on the lower surface of an otherwise conventional body. The circuit board to which a BGA package is to be mounted is conventionally provided with a matrix of corresponding surface mounted flat pad structures upon each of which is deposited a small quantity of solder. To mount the BGA package to the circuit board, the BGA package is temporarily clamped to the board and the board heated (typically by application of infrared energy), causing the solder to melt, fusing the corresponding surfaces together and yielding a strong mechanical and electrical connection when cooled.
BGA packaging is proving to be a powerful ally in the further miniaturization of computers. However, the circuit boards designed to receive the BGAs are lagging in compactness. The problem centers on how to route the electrical conductors from each BGA pad through the circuit board.
In multi-layer boards, electrical signals are routed from layer to layer by metal-coated vias. Accordingly, a via is required for each pad to communicate electrical signals between the pad to a trace on another layer of the board. At the present time, most conventional circuit board designs employ a matrix of vias that is spatially offset from the matrix of electrically conductive flat surface pads and coupled thereto by short traces.
In connecting a BGA electronic component package to the circuit board, the BGA package is typically placed on the appropriate side of the circuit board, using a high accuracy "pick and place" machine, in a manner such that the ball shaped lead portions of the BGA package contact the flat, surface mounted solder pads. The partially completed circuit board/BGA package structure is then subjected to an infrared solder reflow process to mechanically and electrically couple the surface pads to the ball shaped lead portions of the BGA package.
Despite the fact that the ball-shaped configurations of the BGA lead portions allows for a certain degree of self-centering due to surface tension during the solder reflow process, there are practical limitations to the maximum tolerable centering offset between the pads and BGA leads. Additionally, this desirable self-centering effect provided by the ball-shaped configuration of the BGA lead portions tends to be substantially diminished by even slight nonplanarities in either or both of the circuit board and BGA package. These positioning offset and planarity problems, of course, tend to significantly increase the probability of having to rework a significant portion of the fabricated circuit board/BGA package assemblies, thereby undesirably increasing assembly yield and cost.
In addition, as is well known in the circuit board art, BGA packages that are soldered onto printed circuit boards using standard surface mount technology are difficult to rework. Moreover, the fact that these soldered BGA package structures as a practical matter cannot be removed by the end-user precludes this surface mount solder technology from being utilized to package devices that have an upgrade path. These limitations exist, of course, with single chip packages with BGA mounting arrays on their undersides. Additionally, they are present, and even more undesirable, in more complex and expensive laminated multi chip module (MCM-L) packages having BGA mounting arrays built onto the undersides of their substrate portions.
As can readily be seen from the foregoing, it would be highly desirable to provide improved apparatus and methods for operatively connecting a BGA electronic component structure, such as a laminated multi chip module, to a circuit board in a manner eliminating or at least substantially reducing the above-mentioned problems, limitations and disadvantages typically associated with conventional solder connection apparatus and methods. It is thus an object of the present invention to provide such improved connection apparatus and methods.